The present invention relates to a device for use in combination with a radiant burner which stops automatically the flow of fuel to the burner in the event of ignition at the rear of the burner in the fuel distribution chamber thereof.
Radiant burners are used, for example, in drying installations or in installations for correcting the humidity profile of a paper or textile sheet in continuous movement.
The general principle of radiant burners is to convert most of the energy supplied in convective form into a radiant energy as a result of the combustion of a gas.
It should be noted that, for safety reasons, most modern radiant burners operating with gas are of the separate air/gas type. The air and gas are mixed at the last moment just downstream of the radiant burner in the mixing chamber. A radiant burner of this type therefore comprises an air supply, a gas supply, a mixing chamber or tube, a mixture distribution chamber and a combination head or support.
At the outlet of the distribution chamber, they have one or more plates which are made of refractory material of good thermal insulation (ceramic, ceramic or metal fiber, etc.) and the function of which is, on the one hand, to support the combustion and, on the other hand, to convert into infrared radiation of the fraction of the combustion heat recovered as a result of contact with the hot gases.
There can if appropriate, be associated with this plate or these plates a screen which recovers some of the enthalpy still contained in the combustion products and which converts it into additional radiant energy.
Combustion therefore takes place in the region of the plate, more specifically on its front surface, after the gaseous mixture has flowed the latter.
One of the conventional faults of a radiant burner is the phenomenon known as combustion burnback or rear ignition. During such a phenomenon, combustion occurs in the distribution chamber at the rear of the plate, whereas it should normally take place on the front face.
There are various causes of ignition and these can be classified in three categories.
The ignition can be brought about by a combustion burnback through the front face of the plate. In some instances, in fact, combustion can burnback through the plate and take place at the rear of the latter, for example because of an excessive increase in the surface temperature of the plate attributable to, among other things, the presence of a reflector facing the burner, to the partial or complete shut-off of the surface of the plate, to an excessive preheating of the mixture, to an oxygen enrichment, to particular cycling configurations during which the radiant burner is modulated in terms of temperature and power, etc. This combustion burnback through the front face of the plate can also be brought about by defects of the latter, for example by a poor homogeneity or the aging of its component material (in respect of the porosity, thermal conductivity, etc.).
Another cause of ignition is leaks at the rear of the radiant burner. In the event of leakage, the mixture will burst into flames on contact with the hot walls of the burner, and in some instances the combustion will burnback through the leak-producing orifice to the rear of the plate of the radiant burner. There can be many causes of leakage to the rear, for example wearing of the gaskets after aging at the end of the lifetime of the radiant burner, manufacturing defect, etc.
The ignition of the mixture at the rear of the radiant burner can also be brought about by bringing the mixture in contact with a hot spot or a spark before it is introduced into the head of the radiant burner. In fact, some ignition systems of radiant burners comprise an ignition electrode fed by means of a branch connected laterally to the mixer tube, and if the ignition spark is not produced at the appropriate place, namely at the end of the pilot flame, the ignition can be transmitted to the interior of the mixer tube.
Now the consequences of ignition at the rear can be serious if the burner affected is not stopped quickly. In fact, the entire power will be released at the rear into the distribution chamber which will be heated considerably up to 800.degree. to 1000.degree. C. The rear environment (wiring, etc.) therefore risks being damaged, and fires can be caused if inflammable materials are present in the vicinity (papers, plastic films, etc.). Finally, after this operating state has persisted for a few days, the casing of the distribution chamber of the radiant burner risks opening, thereby exposing the flame.
In the traditional systems for assembling radiant burners in an arch or panel-mounted, the burners were accessible from the rear. It was therefore possible, during normal maintenance operations, to identify the defective radiant burners quickly and, if necessary, to change them.
Increasingly now, completely encased batteries of radiant burners are being produced, and the visual detection of faults is consequently difficult, if not impossible.
It is therefore necessary to develop detection techniques which do not require a direct view of the rear of the radiant burners. In particular, existing solutions employ temperature probes or infrared detectors. Particularly, temperature probes are installed on the casing of the distribution chamber and are connected by wiring to an electronically controlled solenoid valve either arranged directly on each of the burners or connected to the existing solenoid valve controlling the complete row of burners.
The solutions described above have some disadvantages. They are generally heavy and costly systems requiring the installation of special wiring and electronics. In the simplest instance of connection to the existing solenoid valve of the row radiant burners, in the event of detection it is necessary to cut off an entire row of radiant burners while waiting for the replacement of the defective radiant burner. Now it is quite clear that the complete shutdown of a row of radiant burners for the repair period is detrimental to the performance of the installation.
The document DE-A-1,526,013 describes and illustrates a mechanical device for shutting off the gas supply port of a radiant burner of the type possessing an element with a controlled fusion temperature which is arranged upstream of the mixing chamber and which is connected to a shutter, the closing element of which is stressed elastically in the shut-off direction and is retained in the open position by the element having a controlled fusion temperature.
This device is effective, but it acts only when a flame flashback occurs and reaches the mixing chamber. For this, it requires a complex arrangement, the purpose of which is to stabilize the flame flashing back into the mixing chamber in order to cause the fusion of the element. This arrangement is complex to produce and unreliable insofar as it is based on a balance of the velocities of the gas flow and of the flashback flame.